1. Field of the Invention
The present invention relates to a vertical alignment liquid crystal display device which is driven by multiplex driving.
2. Description of the Background Art
A liquid crystal display device is broadly used as an information display unit in various electronic devices for consumer or automobile use. In General, a liquid crystal display device is configured by disposing a liquid crystal layer made of a liquid crystal material between two substrates that are placed opposite each other with a gap of roughly several micrometers. As one type of this kind of liquid crystal display device, a vertical alignment liquid crystal display device is known. A vertical alignment liquid crystal display device comprises, as its main components, liquid crystal cells of a VA mode in which liquid crystal molecules are aligned substantially vertically relative to the surface of the respective substrates within the liquid crystal layer that are disposed between the two substrates, and polarizers that are respectively provided to the outside of the liquid crystal cells. The respective polarizers are often provided in a cross-Nicol arrangement. Based on the foregoing arrangement, since the transmittance of the liquid crystal display device during the non-application of voltage will be extremely low, it is possible to realize a high contrast relatively easily.
In order to realize the liquid crystal cells of a VA mode, it is important to control the alignment of the liquid crystal molecules of the liquid crystal layer by performing predetermined alignment treatment to the substrate surface. As for the alignment treatment, for example, treatment (so-called oblique deposition) of forming a thin film having a sawtooth surface on the substrate surface by evaporating metal oxides such as SiOx from a direction that is inclined from the normal line of the substrate, treatment (so-called photo alignment, treatment) of depositing an organic alignment film material such as polyimide on the substrate surface and thereafter irradiating ultraviolet rays from a specific direction, and alignment treatment of forming a vertical alignment film, which has specific surface free energy, on the substrate surface, and performing rubbing treatment thereto are generally known. According to the foregoing alignment treatments, it is possible to obtain an alignment (so-called mono-domain alignment) in which the liquid crystal molecules in the liquid crystal layer are aligned in a single direction during the non-application of voltage.
In the vertical alignment liquid crystal display device having a mono-domain alignment as described above, the setting of a pretilt angle in the liquid crystal layer considerably affects the display characteristics. Specifically, upon considering the steepness in the electro-optical characteristics, the optimal state will be when the liquid crystal molecules are aligned perfectly vertical on the substrate during the non-application of voltage; that is, when the pretilt angle is 90°. In particular, when driving the liquid crystal display device according to the multiplex driving method, an effective means for increasing the display capacity (that is, increasing the duty) is to set the pretilt angle to a greater angle. Nevertheless, since it is not possible to obtain the uniformity of the alignment of the liquid crystal molecules during the application of voltage if the pretilt angle is set to 90°, in a practical sense, it is necessary to perform the foregoing alignment treatment.
Japanese Unexamined Patent Application Publication No. 2005-234254 (Patent Document 1) describes that, in a vertical alignment liquid crystal display device, if the pretilt angle is set to be greater than 89.5° (particularly greater than 89.7°), the maximum transmittance tends to drop as a result of following the size of the pretilt angle. Moreover, with respect to this point, Japanese Unexamined Patent Application Publication No. 2008-281752 (Patent Document 2) describes that one reason thereof is that a dark region appears at a part of the display part under driving conditions where the frame frequency is relatively low, and the display consequently becomes non-uniform. As a method of resolving the foregoing display non-uniformity, foregoing Patent Document 2 discloses the setting of the pretilt angle of the vertical alignment liquid crystal display device and the method of setting the frame frequency in accordance therewith in connection with the respective cases of using various multiplex driving waveforms. Specifically, Patent Document 2 describes that the uniformity of display can be achieved by increasing the frame frequency as the pretilt angle approaches 90°, or using a ½ line inverted drive waveform including numerous high frequency components.
Meanwhile, upon applying the technology disclosed in foregoing Patent Document 2 to a case of driving a vertical alignment liquid crystal display device, which has a relatively large number of scanning lines (that is, large duty), by multiplex driving, the drive current needs to be increased since the internal impedance of the liquid crystal display device will decrease. If the required drive current exceeds the current capability of the driver (driving device) for driving the liquid crystal display device, a voltage drop will occur in the course of scanning the transparent electrodes which serve as the scanning line, and, therefore, the applied voltage to the respective pixels will not become uniform, and the display non-uniformity will become conspicuous. For example, with a dot matrix-type liquid crystal display device in which stripe-shaped transparent electrodes are crisscrossed and a liquid crystal layer is provided therebetween, streaky display non-uniformity along the extending direction of one transparent electrode may occur, or variation in the brightness or contrast along the extending direction of the other transparent electrode may occur. In order to eliminate this kind of display non-uniformity, it is effective to increase the current capability of the driver, lower the frame frequency, or use a drive waveform (frame inverted waveform) that does not contain numerous high frequency components. Nevertheless, increasing the current capability of the driver will lead to increased costs, and it is not necessarily a preferable solution.